In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member. The latent image is then rendered visible by the application of electroscopic marking particles, commonly referred to as toner, to the photosensitive member. The visual image is then transferred from the photosensitive member to a sheet of plain paper with subsequent permanent bonding of the image thereto. This bonding of the toner particles onto the paper generally comprises two steps: fusing, in which the toner particles on the paper are partially melted, or otherwise made fluid; and fixing, in which the fluid toner particles are bonded to the paper. In general parlance, however, these two steps are conceptually combined (since, in many common techniques, the two steps occur substantially simultaneously), and the two steps are together known in the art simply as "fusing."
In order to fuse the image formed by the toner onto the paper, electrophotographic printers incorporate a device commonly called a fuser. While the fuser may take many forms, heat or combination heat-pressure fusers are currently most common. One combination heat-pressure fuser includes a heat fusing roll in physical contact with a relatively soft pressure roll. These rolls cooperate to form a fusing nip through which the copy sheet (the sheet on which the document is finally formed) passes.
Although hot-roll fusing is currently the most common method of fusing in commercially-available electrophotographic printing machines today, numerous other fusing techniques are well known in the art. Fusing by heat alone, by exposing the copy sheet to a heat source, was often used in early plain-paper copying machines. While the use of an electrical heating element by itself affords simplicity of design, it tends to be energy-inefficient, and may present a fire hazard if copy sheets are jammed in the fusing station. Another popular technique is flash fusing, in which a copy sheet is exposed to a quick and intense flash which heats only the top surface of the sheet, and more specifically, mainly the relatively dark areas of toner on the sheet. While flash fusing is efficient for directing light energy for the purpose of fusing, the equipment for creating the flashes tends to be expensive and energy-inefficient. Finally, another common technique is cold pressure roll fusing, in which no external source of heat is used, and the fusing and fixing is carried out by extremely high physical pressure on the sheet. This technique has the advantages of consuming little power, and not requiring any warmup time, but has the disadvantages of creating glossy images and providing a poor fix on solid areas of an image.
Another important technique for fusing is chemical vapor fusing. In this technique, toner on the surface of a copy sheet is made fluid by exposure to a gaseous solvent. Typically, a sheet to be fused is placed on a wireform tray and inserted into a chamber, where fumes evaporate from a wick soaked in solvent. Chemical vapor fusing is most often used in situations where high temperatures are to be avoided and thermal fusing would damage the copy sheet. The copier known under the trade name XEROX 6500, for example, includes a chemical vapor fusing system for use with transparencies. Recent prior art making use of chemical vapor fusing includes U.S. Pat. No. 5,014,090 to Santilli. However, many vapor solvents, such as halogenated hydrocarbons, will emit dangerous fumes or become explosive in a high-temperature environment.
No matter which type of fusing is used in an electrophotographic apparatus, fusing is one of the most constraining parameters in the design of any system. Heat-generating fusers consume from 55 to 70 percent of a machine's power during warmup, and require most of the warmup time. Cold roll fusers and flash fusers require large volumes of space in a machine. In hot roll or cold roll fusing, the dwell time of a copy sheet through the fuser is one of the most important limits to the speed of the machine. The fuser is often responsible for most of the environmental problems of a machine, such as noise, heat, and odor. Finally, the fusing step is one of the most crucial in regards to final copy quality. Improper fusing can cause smearing, lack of uniformity of an image, and/or unattractive mottled appearance to an image. For these reasons, designers of copying machines and printing systems require a great flexibility in selecting which type of fuser they wish to use.
It is an object of the present invention to provide a novel method for fusing in a xerographic system.